1. Field of the Invention
The present invention relates to an improved cadmium zinc telluride (CZT) detector for detecting x-rays or gamma rays and, more particularly, to an improved CZT detector that is useful for detecting x-rays or gamma-rays in a high temperature environment such as an oil well borehole.
2. The Prior Art
In nuclear borehole logging, e.g. in gamma ray-based density and lithology logging, it is important that the gamma ray detectors employed be able to operate at the high temperatures typically encountered in boreholes without undue noise or loss of energy resolution. Borehole temperatures of 150.degree. C.-175.degree. C. and above are not uncommon. Thus historically scintillators and gas tubes have been used as x-ray and gamma ray detectors in nuclear logging tools because of their superior high temperature performance, notwithstanding that such detectors are inferior to solid state detectors (e.g., germanium, silicon and cadmium telluride) in energy resolution, size and noise. This is principally because in most borehole applications, detector noise, dominated by detector leakage current (which is highly temperature sensitive), limits the usefulness of solid state detectors. The use of solid state detectors in borehole logging tools, therefore, has heretofore required a dewar and/or other cooling devices to hold the detector at a low operating temperature to maintain energy resolution and low noise. This cooling requirement substantially offsets any advantage of size afforded by solid state detectors, and further has a limiting effect on the useful lifetime of the detector in the borehole. There has long existed, therefore, a need for a solid state x-ray or gamma ray detector that operates efficiently at high temperatures over a sustained time period without the requirement for cooling.
CZT is a relatively new, solid state, gamma ray/x-ray detector material that is marketed commercially by Aurora Technologies Corporation. of San Diego, Calif. It is grown by a high pressure, vertical Bridgman technique which yields boules of approximately four inches diameter and length. CZT is a ternary compound with a variable stoichiometry and is usually designated by the general formulae Cd.sub.1-x Zn.sub.x Te, where x represents the zinc fraction of the material. For x.apprxeq.0.20, the CZT detector marketed by Aurora Corporation has a bandgap of 1.53 eV, an effective Z of 48, a density of 5.806g/cm.sup.3, and a bulk resistivity of .apprxeq.10.sup.11 .OMEGA.-cm at 25.degree. C. These properties compare favorably to those of cadmium telluride (CdTe): bandgap of 1.47eV, effective Z of 50, density of 6.06g/cm.sup.3 and bulk resistivity of .apprxeq.109. The density and effective atomic number are slightly lower for CZT than for CdTe, but a major advantage of CZT is that its bulk resistivity is two orders of magnitude higher than that of commercially available CdTe.
CZT has received attention from workers in the room temperature, solid state detector industry, such as, for example, for-use in x-ray imaging. Heretofore, however, no attempt has been made to use CZT detectors in harsh, high temperature environments such as oil well boreholes.